About Alternator Testing

About Alternator Testing:

What test can you do on an alternator? You will find all of these “How To” in my new book: “The Hands on Vehicle Testing Guide”.

            How to quickly find out if the alternator is charging.

            How to find out how much the alternator is producing.

            How to test for shorted alternator diodes with alternator in the box (at the

           parts store), on the bench, or in the vehicle with NO wires attached.

            How to test for shorted alternator diodes with alternator in the vehicle with

           the rotor feed wire attached and the B+ stud wire disconnected at the


            How to test for AC riding on DC due to bad alternator diodes.

            How to test for alternator undercharge.

            How to test for alternator overcharge.

            How to test for the alternator charging voltage.

            How to test for intermittent overcharge or undercharge condition.

            How to see the alternator ripple with Vat 40 or Oscilloscope

            How to test for voltage drop between the B+ stud of the alternator

and the battery positive (+) terminal.

            How to do a voltage drop test of the alternator ground side.

In 1996, the term “generator” replaced the term “alternator” in engineering jargon.  I will use the terms interchangeably.  I can remember polarizing a generator.

If you think you have problems with the alternator, don’t even consider looking at it until you have tested the battery. This will assure you that you will not be testing the alternator using a sulfated battery. If a battery is sulfated, the alternator cannot charge it.

When beginning to test the alternator, think basics first. Is the belt loose? Is the alternator mounting bracket loose?


Note:  If you are working on a 1990 to 2007+ GM car or truck, refer to: GM bulletin #43-64-07A regarding low voltage readings or dim lights at idle. Don’t waste your time trying to fix something that, according to the manufacturer, is not really a problem.

The alternator cannot work as engineered if there is a significant voltage drop in the feed wire between the alternator and the battery positive (+) terminal, or between the alternator case, its ground back to the battery negative (-) terminal.

There have been cases where the alternator was replaced with a new one and the new one did not work.This is where voltage drop testing becomes very helpful in a diagnosis.

How can an alternator fail to produce electricity?  Here are the most common ways: 

  1. Belt is not tight enough.
  2. No voltage feed into the rotor.
  3. Voltage regulator is faulty.
  4. Voltage regulator is good, but the grounding circuit for voltage

    regulation is faulty.

  1. Diodes are shorted – See “How components fail”.

The greatest percentage of problems with any alternator is with the diodes.  An alternator with a faulty diode can put out enough current to supply the ignition system, but not enough to keep the battery charged. Diodes that go “bad”, (open or shorted) can be the result of improper test procedures, removing the alternator leads while the engine is running, reversing the battery connections, and from being mishandled.

Ninety-nine out of one-hundred diodes will short, rather than open, when they fail. Shorted diodes in the alternator provide a direct “short to ground” for

battery current. Shorted diodes can discharge a battery in a short period of time,

and cause the alternator to whine or hum at idle or low speed. The whine or hum happens because the alternator, a three-phase device, is “out of phase”. 

Shorted diodes can cause AC to ride on the DC. The vehicle electrical system relies on direct current. The vehicle electronic system relies on direct current, and also on some very specific input of analog signals that can give an appearance of AC.  If a diode does not rectify the AC in the alternator to DC, there will be an excessive amount of AC “riding on DC” throughout the entire electrical system. Voltage and current produced in the alternator is “analog” in nature, looking sometimes much like the signal produced by a crank sensor, wheel speed sensor, etc. 

When AC rides on DC the AC “analog”, or constantly changing signal appears everywhere that voltage is present in the entire electrical/electronic system of the vehicle. This unruly signal has no defined target, like the wires from a permanent magnet generator, but rather is everywhere voltage exists.

Circuits that normally interpret and process analog signals can process the “AC riding on DC”. This causes electrical interference in the computer system. This could cause any control module to be triggered by an unwanted AC signal from the alternator rather than from a sensor engineered to send a specific signal at a specific time, on a dedicated wire. The end result could be a drive-ability problems, problems with cruise control, no start, etc.